The present invention relates to linear motors and, more particularly, to high power linear motors having a design yielding high efficiency by means of a compact coil structure.
Linear motors are used in systems for positioning and moving items, including machining and gantry type systems. The systems often require that items having large masses be subjected to high accelerations. This, in turn, requires that the linear motors exert large continuous forces upon the items to be moved. A large amount of heat is produced by resistive losses in the coil windings during the process of generating such forces. This heat must be dissipated in order to prevent damage to the motors. Forced air or water cooling is often employed to remove the heat. The capacity of the cooling method used limits the power dissipation capacity of the motor.
One of the factors which determines the capacity of the cooling method is the thermal conductance of the coil assembly. This thermal conductance is maximized when the thermal path leading away from turns of the coil is composed of uninterrupted high thermal conductance material. Conventional linear motors utilize coils composed of turns of round cross section wire which inevitably leave insulating air spaces between layers of turns which in turn reduces the thermal conductance of the coil; ultimately restricting the operating power of the motor.
The present conventional design of linear motors employs overlapping coils wound on teeth of a laminated steel core. The force generated by a linear motor is proportional to the area occupied by the steel core. Typically, a force of three to four pounds is generated per square inch of motor area. Therefore, an application requiring 1,000 pounds of force requires 250 to 330 square inches of area, and such a machine would have typical dimensions of 7" by 36" to 48". The large area occupied by such a machine presents a problem in applications having either size or weight restrictions.
Additionally, conventional linear motors generally employ a three-phase driving system. In such a system, individual coils are each driven by one of the phases while overlapping neighboring coils driven by the other two phases. Thus, depending on a degree of overlap involved, the centers of the flux distributions, of three coils driven by the three phases, may be located within a length equaling the width of a single coil. This overlapping requires that the windings overlap each other outside of a toothed portion of the core. This overlapping results in a bulky arrangement of the coils outside of the core and a further increase in the space requirement of the linear motor.
The material employed in the construction of the core of such machines is generally selected from core materials, such as steel alloys, having a high magnetic saturation level and a high magnetic permeability in order to permit the production of the high magnetic flux densities required for the production of large forces by the linear motor. Such core materials are expensive. Therefore, the required size dictates that the costs of the materials significantly contribute to the cost of the linear motor.
In the conventional linear motor, permanent magnets attached to a back iron plate above the coils of the armature are generally rectangular in shape. These magnets are mounted next to each other, each successive magnet having a pole orientation opposite that of the prior magnet. The magnets are aligned so that they are inclined a slight angle from a normal to an axis of movement of the linear motor. This inclined angle creates a flux distribution along the axis of movement which is generally sinusoidal in nature. Such a distribution reduces a cogging effect in the operation of the linear motor which would otherwise occur if the magnets were aligned normal to the axis of movement.
Although the inclined angle of the magnets reduces cogging, it presents a disadvantage in that a larger area must be covered by the rectangular magnets in order to sufficiently cover and interact with the coils of the armature. Portions of the rectangular magnets must protrude out past the ends of the coils so that coverage is achieved. This results in wasted magnet material, increased size, weight and cost.
Furthermore, since each magnet is mounted next to the adjacent magnet with little space therebetween, holes through which the back iron may be mounted must be located outside the area of the magnets. Protruding ears with holes therein are used to mount the back iron. Such a method increases the footprint size of the back iron and its weight. Both increases present problems in applications were there are size constraints and low mass is desirable. If the back iron plate is mounted on the movable portion of the linear motor, the increased mass reduces the acceleration achieved by the motor and the ability of the motor to stop thereby requiring an increase in the dissipation of power.